How did it get to the point where walking down a city street literally comes with a health warning?

Imagine. It's tomorrow morning. You wake up, stagger to the kitchen, and pour a glass of water from the tap. It tastes slightly bitter, so you take a closer look. It's cloudy, murky. You switch on the radio and hear that the drinking water in London is contaminated by a poisonous toxin. It's way past the legal limit for safety and damages lungs, hearts, and brains. The country is in uproar. The government escalates the health crisis to a number one priority.

This is fiction, but the reality is not much different. London's air—the stuff we breathe every day—is toxic, but the official reaction is, weirdly: meh.

In January, London's air pollution rose above Beijing's levels, leading to a "very high" alert for the first time. Headteachers wrote to Mayor Sadiq Khan, asking him to act on his promises to clean up the air. In November 2016, the courts ruled for the second time that the government's air pollution plan is illegally bad.

"Living in a place with long-term air pollution problems has significant impacts on your health," says Dr. Ian Mudway, lecturer in Respiratory Toxicology at King's College London. But it's not only the dangerous spikes that impact our health; the long-term consequence of breathing polluted air will get worse as you get older.

London does not fulfill its annual nitrogen dioxide EU limit value, and has breached it for five years. Other major cities—including Leeds, Liverpool, Birmingham, Cardiff, and Edinburgh—also breach NO2 levels.

Toxic air makes symptoms of preexisting lung and heart diseases far worse—if you're asthmatic, for example, your chest may feel tighter in cold, still weather—but even if you don't have health conditions already, the long-term consequence of breathing polluted air has a cumulative effect, and increases vulnerability to disease later in life.

"If you're in your early 20s, you're one of the most vulnerable groups to the effects of toxic air."

Essentially, the state of London's pollution could make our older years suck. As Mudway says, when you get old, you want to be healthy. "It's not just about losing a short period of life at the end of life; it's about your quality of life as you age."

If you're in your early 20s, you're one of the most vulnerable groups to the effects of toxic air. Evidence suggests that it can affect the growth of your lungs—which only reach their final capacity at 23 for men and 20 for women. As lung volume decreases through age, you become more vulnerable to chronic respiratory conditions.

The dirty air could even have effects on our brains. "There may be effects of air pollution on cognitive development in children, and also potentially impacts on degenerative disease in the elderly," says Mudway.

In the 1990s, diesel cars were promoted as "greener" than gasoline, but concentrations of nitrogen oxide and other greenhouse gases increased. In September 2015, it was discovered that VW was cheating emissions tests. Around 11 millions cars were fitted with devices which allowed them to produce false results. People thought they were buying environmentally friendly cars when in fact the engines were belching out gasses which amounted to, as German chemist Axel Friedrich put it, "physical assault." Later, it was found that the leniency of diesel emissions-testing was allowing other car manufacturers to sell vehicles that broke pollution rules.

"In the UK, we've seen very little done to car companies since 'dieselgate' erupted," says Greenpeace's Ellen Booth. The expansion of the UK's motorways and the new Heathrow runway show that cleaning up our air isn't a priority of the British government, she says.

Other capital cities have taken action. Paris has banned diesel cars manufactured before 2000. Between 6:30 PM and 9 PM in Madrid, even-number registration plates are only allowed to drive on even-number days, and cars with odd-number registration plates on odd-number days.

Transport for London is planning an Ultra Low Emission Zone—an area stretching from Marble Arch to Tower Hamlets—where vehicles will need to meet exhaust emission standards or pay a charge. But it won't come into force until 2020. Sadiq Khan is trying to bring it forward to 2019.

"The [anti-pollution] advice isn't targeted to the polluter."

So what can you do? The government's current health advice is incredibly helpful: If you have lung problems or are old, stay inside! Avoid busy roads. And don't bother with a mask—they don't work because NO2 particles emitted at the roadside are so tiny.

"The advice isn't targeted to the polluter," says Medway. There are hardly any incentives to encourage drivers to stop using their cars. "The greatest thing an individual can do is not contribute to it," says Mudway. Walk, cycle, or use sustainable transport. Think about what you're ordering online—a rise in Amazon deliveries and Deliveroo orders contributes to the problem.

At least we're now warned when pollution is high. "We used to be sitting here looking at air pollution episodes which were at the level where people with respiratory and cardiovascular conditions should be taking steps to protect their health, and there was silence," says Mudway.

Think of city smog, and you might think of the 1950s pea soupers or the soft black drizzle of Dickensian London. But the insidious and invisible cloud we're now living in is the largest environmental health crisis the world is facing, according to WHO. So instead of breathing our way into a slow, painful, death, it's time to start caning our MPs with letters asking the question: Why the fuck is the UK's reaction so slow?